1. Field of the Invention
The present invention relates to a carbon film to be formed on the surface of a soft base material such as rubber or plastic (for example, such a carbon film having characteristics as low friction coefficient or low wear, to be used to such as sliding portion of machine parts), and a method of forming the same.
2. Description of the Related Art
Being high in hardness, carbon film is much used to cover the surface of metallic base material demanded to have high hardness as tools or parts of vehicles.
The carbon film is also sometime covered on the surfaces of soft base material such as rubber or plastic. In these cases, if covering the carbon film on the surface of soft base material, it has a merit to decrease frictional resistance on the surface and provide high hardness as maintaining the flexibility of soft base material. For this fact, recently studies and developments have rapidly been advanced for covering the carbon film on the surfaces of soft base material.
For covering the carbon film on the surface of soft base material, for example, the soft base material is located within a closed film forming container, a gas containing carbon is introduced into the container, the gas is made plasma by use of means for making the gas plasma (for example, means as a high frequency discharge), and a carbon film containing carbon as a main component is formed on the surface of soft base material under the plasma.
The soft base material with carbon film formed thus is often used as a sliding member. For example, the soft base material covered with the carbon film is slid together with other article under pressure.
However, the sliding member composed of the soft base material with the carbon film is used under various circumstances. For example, in the sliding member, a predetermined pressure is loaded on the surface of the soft base material with the carbon film, and at this time, friction coefficient of the carbon film is demanded to be low for maintaining a function as the sliding member. For example, when pressure added to the surface of the carbon film on the soft base material is below a certain value, the friction coefficient of the carbon film is low and desirable; however, if being under pressure larger than this value, the friction coefficient of the carbon film is sometimes high, so that the function as the sliding member cannot be maintained.
Further, even if the carbon films are subject to the same pressure, there are cases that the friction coefficient is controlled to be low and controlled to be high.
Accordingly, it is an object of the invention to provide a carbon film containing carbon as a main component, formed on the surface of a soft base material, enabling to control friction coefficient to be low, and a method of forming this carbon film.
It is another object of the invention to provide a carbon film containing carbon as a main component, formed on the surface of a soft base material, where friction coefficient is controlled to be low in response to pressure to be applied to the surface of the carbon film.
It is a further object of the invention to provide a method of forming the carbon film containing carbon as a main component, formed on the surface of a soft base material, where friction coefficient is controlled to be low in response to pressure to be applied to the surface of the carbon film.
An inventor made studies for solving the problems, and obtained the following findings.
Namely, the carbon film containing carbon as the main component, formed on the surface of soft base material is divided by cracks into a plurality of regions. The carbon film may be produced by, for example, the following carbon film forming apparatus.
FIG. 3 of attached drawings shows an existing carbon film forming apparatus by Plasma CVD Process where methane gas is a raw gas.
This carbon film forming apparatus comprises a film forming container 1, a vacuum exhaust apparatus 2, a shower plate 3, and means for making plasma the gas (herein, a high frequency electrode 5 connected to a high frequency power source 8 via a matching box 7). The vacuum exhaust apparatus 2 enables to exhaust an air from an interior of the film forming container 1. The shower plate 3 introduces a gas containing carbon (herein, methane gas) into the film forming container 1 vacuum-exhausted by the vacuum exhaust apparatus 2. The means for making plasma the gas makes the gas introduced into the film forming container 1 through the shower plate 3 plasma. Incidentally, the high frequency electrode 5 may mount the soft base material 4 thereon. The high frequency electrode 5 is impressed the high frequency voltage from the high frequency power source 8 via the matching box 7. Thereby, the gas introduced into the container 1 is made plasma which is to be formed at a position shown with numeral 6 in the drawing.
In the carbon film forming apparatus shown in FIG. 3, the soft base material 4 is located on the high frequency electrode 5 in the film forming container 1, and the interior of the container 1 is closed. While the interior of the closed container 1 is vacuum-discharged by the vacuum exhaust apparatus 2, the methane gas (CH4) is introduced as the raw gas from the shower plate 3, and the interior of the container 1 is maintained at a desired film forming pressure. In general, the pressure within the vacuum container 1 ranges 0.1 to 10 Pa. The high frequency electrode 5 mounting the soft base material 4 thereon is impressed with the high frequency voltage. Then, a methane gas plasma 6 is formed within the film forming container 1, and carbon being a composing element of methane is deposited on the soft base material 4. Thus, the carbon film is formed on the soft base material 4. By the way, the methane gas is employed as the raw gas, and when the carbon film is formed on the soft base material, benzene gas (C6H6) is generally used other than the methane gas.
FIG. 4 shows a schematically enlarged view of the surface condition of the carbon film grown by use of the methane gas or benzene gas.
Observing the surface of the grown carbon film by means of such as electron microscope, it is seen as shown in FIG. 4 that numberless cracks A appear in the film, and the film is finely divided into a plurality of regions B due to cracks A. Herein, each of regions B surrounded by cracks A in the carbon film shall be called as xe2x80x9cblockxe2x80x9d.
In a conventional film forming process of the carbon film employing the methane gas or benzene gas as the raw gas, when forming the carbon film on the soft base material, if changing conditions such as an electric power supplied in the formed film, the average area of the block B may be changed. When employing the methane gas as the raw gas, the average area of the block B ranges around 0.01xc3x9710xe2x88x923 mm2 to 0.1 to 10xe2x88x923 mm2, and when the benzene gas is the raw gas, it ranges around 0.06xc3x9710xe2x88x923 mm2 to 0.12 to 10xe2x88x923 mm2.
According to the inventor""s studies, it was seen that if using a gas containing carbon as a raw gas to be introduced into a film forming chamber, which is higher molecular than that of benzene (for example, a hydrocarbon higher molecular than such as benzene), otherwise if using a gas containing oxygen atom and including carbon, or using a gas containing oxygen atom, which is added to the gas containing carbon, the average area of the block B might be controlled, so that the carbon film of larger average area of the block B might be obtained.
The inventor made further studies to investigate the relationship between the average area of the block B and the friction coefficient of the carbon film. It was found that a close correlation was present between them. That is, when the average range of the block B was smaller than around 0.15xc3x9710xe2x88x923 mm2, roughness in the surface of the carbon film was large. When it was larger than around 20xc3x9710xe2x88x923 mm2, the carbon film was finely cracked under pressure in the respective blocks B, resulting to be the surface roughness in the carbon film. In either way, the friction coefficient of the carbon film tended to be large. However, if the average area of the block B was around 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2, the friction coefficient of the carbon film could be controlled to be low. In other words, it was found that if the range of the average area of the block B was selected from around 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2 in response to the pressure to be applied to the surface of the carbon film, the friction coefficient of the carbon film might be controlled to be low.
In the conventional carbon film forming process using the methane gas or benzene gas as the raw gas, since the controlling range of the average area of the block B is narrow as 0.01xc3x9710xe2x88x923 mm2 to 0.1xc3x9710xe2x88x923 mm2 in the case of the methane gas, and 0.06xc3x9710xe2x88x923 mm2 to 0.12xc3x9710xe2x88x923 mm2 in the case of the benzene gas, it is not possible to fully take out the effect of controlling the friction coefficient of the carbon film to be low. However, if using a gas containing carbon as a raw gas to be introduced into the film forming chamber, which has a higher molecular weight than that of benzene (for example, a hydrocarbon higher molecular than such as benzene), otherwise if using a gas containing oxygen atom and including carbon, or using a gas containing oxygen atom, which is added to the gas containing carbon, it is possible to obtain a carbon film having a larger average area of the block (that is, the average area of around 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2). Accordingly, it is possible to obtain such a carbon film where the friction coefficient is controlled to be low.
Based on the above mentioned findings, the invention is to provide the following carbon film and the method of forming the carbon film.
(1) The Carbon Film
A carbon film containing carbon as a main component, formed on the surface of a soft base material, wherein the film is cracked and divided into a plurality of regions, and an average area of respective regions surrounded by cracks is 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2.
(2) The Method of Forming the Carbon Film
A method of forming a carbon film, wherein a soft base material is located within a closed film forming container, a gas containing carbon is introduced into the container, the gas is made plasma by use of means for making the gas plasma, and a carbon film containing carbon as a main component is formed on the surface of a soft base material under the plasma, said film being cracked and divided into a plurality of regions, and having an average area of respective regions surrounded by cracks being 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2.
In the carbon film and the method of forming the same according to the invention, the average area of the respective regions (block) surrounded by cracks are obtained as follows.
Namely, with respect to each of five parts of the same area (500 xcexcmxc3x97500 xcexcm=0.25 mm2) arbitrarily selected in the carbon film, enlarged observation of 250 magnifications by SEM (the scanning type electron microscope) is carried out to count piece number of blocks in the area, and the area (0.25 mm2) of the each part is divided by the piece number of the block so as to demand the block area, and as to the thus demanded five parts, the average value of the block area is obtained to be average area of the block.
According to the carbon film by the invention, since the average area of the respective regions surrounded by cracks is 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2, the friction coefficient of the carbon film is controlled to be low. To further say, the friction coefficient may be controlled to be low also in a mutual dry-sliding between the carbon film and other article, not using lubricant as an oil.
The soft base material may be such a material to be slid together with other article under pressure or under a dried condition not interposing the lubricant as an oil. In this case, as optimum ranges of the average area of the respective regions surrounded by cracks where the friction coefficient of the carbon film is controlled to be low in response to pressure to be applied to the surface of the carbon film, there are exemplified around 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2 when the soft base material is to be slid together with the other article under pressure below 8xc3x97105 Pa; around 0.15xc3x9710xe2x88x923 mm2 to 4xc3x9710xe2x88x923 mm2 when the soft base material is to be slid together with the other article under pressure from 8xc3x97105 Pa to 4xc3x97106 Pa; and around 0.15xc3x9710xe2x88x923 mm2 to 2xc3x9710xe2x88x923 mm2 when the soft base material is to be slid together with the other article under pressure above 4xc3x97106 Pa.
The method of forming the carbon film according to the invention can produce the carbon film of the invention. As to the carbon film produced by the method of forming the carbon film according to the invention, the same as mentioned above concerning the carbon film of the invention may be referred to.
In the film forming method of the invention, the soft base material is located within the closed film forming container, the gas containing carbon is introduced into the container, the gas is made plasma by use of means for making the gas plasma, and such a carbon film is formed on the surface of the soft base material under the plasma, that the average area of the respective regions surrounded by cracks is 0.1xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2.
According to the carbon film forming method of the invention, as the carbon film to be formed has the average area of the respective regions surrounded by cracks being 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2, it is possible to obtain the carbon film of the friction coefficient being controlled to be low.
The carbon film forming method of the invention may employ any in the following exemplified gases in (a) to (c), whereby it is possible to obtain the carbon film having the average area of the respective regions surrounded by cracks being 0.15xc3x9710xe2x88x923 mm2 to 20xc3x9710xe2x88x923 mm2.
(a) As examples of hydrocarbon gas having higher molecular weight than that of benzene, exemplified are gases, molecular symbols of which are expressed with CxHy (Xxe2x89xa76 and Yxe2x89xa77), for example, C6H12 (cyclohexane) gas, C6H14 (hexane) gas or C8H18 (octane) gas.
(b) As examples of gases containing oxygen atom and carbon atom, gases expressed with molecular symbols of CxHyOz may be exemplified. Herein, x, y, z are respectively integers or 1 or more. As more actual examples, exemplified are xxe2x89xa71, yxe2x89xa72 and zxe2x89xa71. Practically, CH3OH (methanol) gas, C2H5OH (ethanol) gas and CH3COCH3 (acetone) gas are exemplified.
(c) As an example of a gas containing oxygen atom, which is added to a gas containing carbon, there are exemplified a mixture of a raw gas containing carbon and a gas of a main component being metallic oxide or a gas of a main component being silicone oxide. Practically, a mixture of CH4 (methane) gas or C6H6 (benzene) gas and a gas of a main component being TiO2 (titanium oxide); or a mixture of CH4 (methane) gas or C6H6 (benzene) gas and a gas of a main component being TEOS (tetraethoxysilane). In this case, as a gas containing carbon, in substitution for methane gas or benzene gas, a gas of CxHy (Xxe2x89xa76 and Yxe2x89xa77) may be employed.
Either way, in the carbon film and the carbon film forming method according to the invention, as the soft base material, for example, a rubber base material such as silicone rubber or plastic base material are enumerated though being not limited to.